The present invention relates to a transmission system using optical fibers and, more particularly, to a long-distance, large-capacity optical communication system employing dark soliton pulses and optical amplifiers and an optical transmitter and an optical receiver for use in the system.
Buttressed by developments of optical amplifying techniques, optical fiber communication technology has made rapid-paced progress toward ultra-long-distance communication, now allowing implementation of a transpacific communication system without the need of using regenerative repeaters. At increased transmission rate, however, conventional transmission systems suffer serious degradation of their transmission characteristics that are caused by the wavelength dispersion characteristic and nonlinear optical effect of optical fibers, imposing severe limitations on the realization of a high-speed, high-capacity transmission system. In recent years, an optical soliton communication system has been in the limelight as a system that will break the bottleneck in the speedup of transmission by the wavelength dispersion characteristic and the nonlinear optical effect.
The optical soliton communication system is a system that positively utilizes the wavelength dispersion characteristic and nonlinear optical effect of optical fibers which are major factors to the degradation of characteristics of the conventional transmission systems and that transmits optical short pulses intact by mutually balancing optical pulse width expansion owing to the wavelength dispersion in the optical fibers and pulse width compression based on the nonlinear optical effect. In case of using, as repeaters, optical amplifiers which compensate for a loss in optical fibers, it is possible to realize soliton communication with practically no waveform variations of optical pulses like ideal soliton pulses, by setting an average power between repeaters and an average dispersion of optical fibers to soliton conditions.
In the optical soliton communication at a high transmission rate of 20 Gb/s or so, optical amplifier noise affects the timing jitter of optical pulses at the receiving end and eventually deteriorates the transmission characteristic. That is, optical soliton pulses with noise superimposed thereon undergo random fluctuations of their optical intensity and slightly departs or differs in shape from ideal optical soliton pulses, causing fluctuations in the shift amount of the carrier frequency by the nonlinear optical effect. Since these operations are repeated for each repeater, the time of arrival of optical pulses randomly fluctuates during their propagation in optical fibers each having a limited dispersion value, incurring the timing jitter at the receiving end. This phenomenon is called the Gordon-Haus effect, which is a major limiting factor to the transmission characteristic of the optical soliton communication. Furthermore, in case of transmission of a plurality of optical soliton pulses carrying digital information, if the repetition period of transmitted soliton pulses is too short, it is observed that adjacent soliton pulses attract or repel each other by virtue of their interactions. This also causes the timing jitter at the receiving end, and hence is not preferable for the application of optical soliton pulses to communications. To suppress the interactions of soliton pulses, it is necessary to widen the space between adjacent soliton pulses to some extent.
With a view to overcoming the timing jitter problem, there have been intensively studied soliton pulse control techniques for artificially reducing the timing jitter, and soliton transmission experiments have made rapid progress in the last few years. There are two approaches to controlling soliton pulses: one is to control a random frequency shift by an optical filter in the frequency domain, and the other is to directly control the timing jitter itself in the time domain. In the prior art, however, these methods involve complex processing using an ultra-narrow optical band-pass filter or an optical modulator in the repeater. This is not desirable from the practical viewpoint such as the long-term stability of the system.
To implement a high-capacity long-distance transmission system employing optical amplifiers, it is important to minimize the complexity of the transmission line including optical amplifiers without providing any special means as in the prior art and to transmit a high-bit-density optical signal with a soliton pulse train of a high bit density.
In the conventional optical soliton communication, there has been used what is so called a bright soliton scheme which transmits short optical pulses in the wavelength band covering an abnormal dispersion range of optical fibers. On the other hand, it is theoretically known that in a case where an ON-OFF reversed signal of optical pulses, that is, a lightwave of a fixed intensity but with a sharp depression in its waveform (dark pulses) is transmitted in the normal dispersion region of optical fibers, if the signal intensity and the pulse width (the width of the depression) of the dark pulses satisfy a certain relationship, the transmission can be achieved with no degradation of the waveform (the shape of the depression) as is the case with the bright soliton lightwave (a. Hasegawa and F. Tappert, Appl. Phys. Lett., Vo. 23, pp. 171-172, 1973); this is called a dark soliton transmission. In this instance, however, it is necessary to provide an optical phase shift at the center of the dark pulses.
FIG. 12(a), (b) and (c) show typical waveforms of bright soliton pulses and dark soliton pulses (J. R. Taylor et al., Optical Solitons-Theory and Experiment, chap. 10, Cambridge University Press, 1992). FIG. 12(a) shows an example of the bright soliton pulse, which has a constant optical phase. On the other hand, as depicted in FIGS. 12(b) and (c), the dark soliton has features in that the optical intensity has an ON-OFF reversed profile of that of the bright soliton and that of the optical phase shifts. FIG. 12(b) shows a case where the optical intensity is zero at the bottom of the depression (black soliton), and in this case, the optical phase shifts by .pi. at the center of the depression. FIG. 12(c) shows a case where the dark pulse goes down to 1/2 of the intensity of CW laser light (gray soliton) and the optical phase shift amount is .pi./2. As shown in FIG. 12, A and B are parameters which represent the depth of the depression and the relative level of background light, respectively; A=B=1 corresponds to the black soliton and B approaches zero as the level of the background light increases. The phase shift amount Ps of the dark soliton is given by the following equation by the use of the parameter B. EQU Ps=2 sin.sup.-1 .vertline.B.vertline.
The dark soliton has an advantage over the bright soliton in that it permits reduction of the Gordon-Haus jitter to about 70% and suppression of the soliton interaction. (Y. S. Kivshar, IEEE J. Quantum Electronics, Vol. 29, pp. 250-264, 1993). In the past, however, there was not available a transmitter having a generator for the bright soliton lightwave added with digital information and a receiver therefor; hence, no attempts have been made to apply the dark soliton to optical communication.